The present invention relates to a reference timing signal oscillator which oscillates in phase-lock with a reference input signal and more particular to a reference timing signal oscillator, the frequency of which is stabilized in the event of no reference input signal. The reference timing signal oscillator is applicable to a cellular telephone base station for example. The present invention is also applicable to reference signal or clock signal generators in other types of apparatus, for example, optical transport networks.
A known cellular telephone system is a Code Division Multiple Access (CDMA) system. The CDMA wireless phone system allows multiple cellular phone users to share the same frequency spectrum, and uses a generated noise carrier with a different and essentially orthogonal instance of the noise carrier assigned to each mobile unit within a cell. The base station receiver in a CDMA station correlates the received signal from a mobile unit with the desired noise carrier, extracting the transmitted digital signal with a sufficient signal-to-noise ratio to achieve a satisfactory data error rate. Because the base stations in a system such as CDMA must be synchronized with surrounding base stations to handle handoff of mobile phones between cells and for other functions, a time reference must be provided to each base station. This is commonly provided via Global Positioning System (GPS) receivers which comprise a part of each base station. GPS satellites each provide radio signals that are synchronized and usable by GPS receivers not only to derive one""s physical position relative to the satellites but also to derive a very accurate time reference. Because the GPS receiver antennas of cellular phone equipment are often placed high relative to surrounding terrain, they are subject to lightning damage in addition to physical damage from rough handling or other damage. CDMA base stations which lose contact with GPS satellites should ideally continue to operate during this holdover period until contact can be reestablished, whether through repair of damaged equipment, or other changed circumstances. A crystal oscillator may provide a time reference during this holdover period, as long as the oscillator is stable enough to keep the base station sufficiently synchronized with other base stations.
A method is needed for improving on the performance of current CDMA base station clock stability when the base station is not receiving a GPS signal to provide a clock reference. When no GPS signal is received the system clock operates in holdover mode, and the clock signal is generated by a crystal oscillator designed to provide a signal of the same frequency as is provided by the GPS receiver. It is necessary to improve current GPS-based clock stability during holdover by compensating for the performance of a crystal used to generate a clock signal during this holdover period.
A typical CDMA base station uses a received GPS signal to produce a reference clock signal to ensure that CDMA stations are synchronized in operation. Such synchronization between stations is important to the proper operation of a CDMA system, as common operations such as a CDMA spread spectrum code search and station-to-station handoff require that stations be closely synchronized in time. Mobile stations also synchronize to the signals provided by the base station, such that the GPS clock provides a timing reference for both the base station and all mobile stations active within the cell.
This synchronization is jeopardized when the base station fails to receive a GPS signal, and must rely on an oscillator to maintain time independent of the GPS signal still used by neighboring base stations. This commonly occurs as a result of lightning strikes that damage the GPS antenna or receiver of a CDMA system, and also occurs as a result of damage due to rough handling and vandalism or from other causes. If the oscillator is not sufficiently stable, the time it provides to the base station may drift with respect to the desired GPS reference time, and cause the base station to fail to communicate properly.
Currently, a new oven-controlled crystal oscillator (OCXO) used to provide a holdover clock signal in a CDMA system is burned in and tested in operation for frequency stability for no more than a few days. Crystals that perform adequately are then accepted for service and placed in use as part of a CDMA base station. But, because crystals often take from 20 to 30 days to settle in or become stable in performance, this test cannot ensure performance of the crystal in extended operation. Excessive frequency drift due to molecular settling or spurious frequency jumps due to contaminants in the crystal may cause the crystal to perform much more poorly in the field than these preliminary tests could indicate. Other factors such as rough handling during installation or spurious mechanical changes in the crystal may further degrade crystal stability, and are not detectable after initial testing. In order to reduce base station cost, lower cost reference oscillator may be used. However, in general, the frequency stability of low cost OCXOs is poor and thus, the frequency stability in the holdover period is not reliable. It must be ensured that the increased frequency drift in the low cost OCXOs used as base station reference oscillators is compensated sufficiently to maintain the required level of base station synchronization.
U.S. Pat. No. 6,194,970 issued to Nielsen et al. on Feb. 27, 2001 discloses an oscillator stability monitoring and compensation system for analyzing the steering voltage applied to a crystal oscillator over time and compensating for spurious frequency jumps in determining the drift rate of a crystal oscillator. The steering voltage is used to estimate oscillator stability by comparing a projected steering voltage against an actual voltage after a simulated holdover period, or analyzing a steering voltage recorded over a period of time and evaluating rates of change. Spurious frequency jumps are removed from data collected while not in an actual holdover, making the data more accurately represent the frequency drift rate of the oscillator. The rate of occurrence of spurious frequency jumps while not in holdover may be monitored to provide information regarding the physical condition of the crystal. However, the system is directed to detect spurious frequency jumps in the crystal and compensate for these jumps in charactering the performance of the crystal. It does not address the problem of a low performance reference oscillator.
U.S. Pat. No. 5,697,082 issued to Greer et al. on Dec. 9, 1997 discloses a self-calibrating frequency standard system self-calibrating a clock of a communication terminal for use with communication systems in which a central communication node generates time base correction signals for the terminal clock includes a terminal oscillator which generates an oscillator frequency that includes an error amount. An oscillator calibration filter generates a frequency error estimate amount. The frequency error estimate amount generated by the calibration filter is subtracted from the oscillator frequency error amount. The time base correction signals are applied to the calibration filter to thereby modify the frequency error estimate amount generated by the calibration filter based upon the time base correction signals generated by the communication central node. It does not address the problem of low performance reference oscillator, either.
For example, in the North American synchronous CDMA, it is required to meet with all reference oscillator specifications with respect to the accumulated timing error over the holdover period. An example of specification for a compact base transceiver system (BTS) is 6.9 microseconds cumulative timing error in 24 hours, which translates to a frequency stability requirement of 0.08 parts per billion on the 10 MHz reference oscillator. It is noted that the holdover specifications include all operating condition variations, the most severe of which from a crystal oscillator design standpoint, is the xe2x88x925 Celsius to +70 Celsius ambient temperature range.
The simplest approach to the problem of determining the oscillator frequency when a more stable reference is unavailable is to avoid the requirement for doing so altogether. In order not to have to know the oscillator frequency during the holdover period, the problem is that one of ensuring the cumulative time error of the free running oscillator remains within the specified 6.9 microseconds. To achieve the required level of stability directly from the oscillator without recourse to any form of control loop places severe demands on the crystal cut and thermal stabilization through the use of double oven architectures. It is, thus, required to improve the oscillator long term stability with low cost. For example, rubidium oscillators and conventional double oven crystal oscillator are still expensive.
It is an object of the present invention to provide an improved reference timing signal oscillator, the frequency of which is stabilized in the event of no reference input signal.
The present invention is directed to a phase-locked loop (PLL) for providing a timing output signal, in one aspect. The PLL comprises an oscillator, a difference detector and a processor. In the PLL, an oscillator generates an oscillation output signal in response to a control component (e.g., voltage) of an input control signal. The difference detector detects a difference between the oscillation output signal and an input reference timing signal, when the input reference timing signal is available. A difference signal is provided, in accordance with the detected difference, to the processor. The processor varies the control component of the input control signal in accordance with the detected difference and a frequency dependent element relating to the oscillator. The frequency dependent element is provided in accordance with a characteristic model. The model is updateable in accordance with the difference detected by the difference detector, when the input reference timing signal is available. The oscillator is controlled in accordance with the control component varied by the processor and the frequency of the oscillation output signal generated thereby is controlled.
For example, in a case where the PLL including the oscillator is used as a reference timing signal oscillator in a cellular base station, the input reference timing signal is available from a cellular receiver when it is locked and the input reference signal is unavailable when signal receiving is interrupted, that is, a holdover period.
For example, the processor includes first and second processing units. The first processing unit provides a first characteristic in accordance with a first parameter of a first model included in the characteristic model. The first parameter is relating to an aging characteristic of the oscillator. The first processing unit updates the first parameter in accordance with the detected difference, when the input reference timing signal is available. The first processing unit updates the first parameter in accordance with its past or preceding parameter, when the input reference timing signal is unavailable.
The second processing unit provides a second characteristic in accordance with a second parameter of a second model of the characteristic model. The second parameter is relating to a temperature characteristic of the oscillator. The second processing unit updates the second parameter in accordance with the difference signal, the first characteristic provided by the first processing unit and a temperature in relation to the oscillator, when the input reference timing signal is available. The second processing unit updates the second parameter in accordance with its past or preceding parameter, when the input reference timing signal is unavailable. The first and second characteristics are combined to produce the characteristic signal to vary the frequency of the oscillation output signal of the oscillator. Since the first and second parameter relates to frequency dependent elements on the frequency of the oscillator, with adaptively updating of the characteristic model, the frequency change to the oscillation signal over time is compensated.
The second processing unit may also update the second parameter in accordance with the detected difference, the first characteristic provided by the first processing unit, a temperature in relation to the oscillator and a voltage in relation to the oscillator. In this processing unit, the frequency of the oscillator is controlled in accordance with the voltage which may affect the oscillation. The temperature and voltage are obtained by their respective sensors implemented into the oscillator.
In a case where the reference timing signal oscillator including the PLL is applied to a base station, for example, it improves current GPS-based clock stability during holdover by better estimating and compensating for the performance of a crystal used to generate a clock signal during the holdover period.
Advantageously, each of the two processing units may include an update determinater for determining whether the respective parameter is updated, regardless of the availability of the input reference timing signal. The determination is made in accordance with a predetermined period, wherein the respective processing unit updates the parameter in accordance with its preceding past parameter. Due to this determination, when the input reference timing signal is available, the first and second processing units update the first and second parameters in accordance with their preceding parameter values, respectively. This is done periodically, in accordance with the oscillation frequency.
According to another aspect of the present invention, there is provided a method for providing an accurate timing output signal when an input reference timing signal is unavailable. In the method, a difference between an oscillation output signal of an oscillator and an input reference timing signal is detected when the input reference timing signal is available to provide a difference signal. A control component (e.g., voltage) of the input control signal applied to the oscillator is changed in accordance with the detected difference and a frequency dependent element relating to the oscillator. The frequency dependent element is provided in accordance with a characteristic model. The model is updateable in accordance with the detected difference, when the input reference timing signal is available. A frequency of the oscillation output signal is controlled in accordance with the varied control component. The timing output signal is provided in accordance with the oscillation output signal.
For example, the characteristic model is adaptively updated in accordance with aging and temperature characteristics relating to the oscillator. Since the first and second parameters of the model relate to frequency dependent elements on the frequency of the oscillator, with adaptively updating of the characteristic model and the parameters, the frequency change to the oscillation signal over time is compensated.
According to another aspect of the present invention, there is provided an apparatus for generating a reference signal for use in a cellular base station, the apparatus comprising: a receiver for generating an input reference timing signal when a cellular signal is available; an oscillator for generating an oscillation output signal in response to a control component of an input control signal; a difference detector for detecting a difference between the oscillation output signal and the input reference timing signal when the input reference timing signal is generated, thereby providing a difference signal; and a processor for varying the control component of the input control signal applied to the oscillator in accordance with the difference detected by the difference detector and a frequency dependent element relating to the oscillator, the frequency dependent element being provided in accordance with a characteristic model that is updateable in accordance with the difference signal provided by the difference detector when the input reference timing signal is generated, a frequency of the oscillation output signal generated by the oscillator being controlled in accordance with the varied control component of the input control signal.
The model updating may be performed by a computer algorithm. With such model updating, oscillators of low stability performance may be used as cellular base station reference oscillator, because high frequency stability is achieved. In order to update the characteristic model, adaptive algorithm may be used. For example, an adaptive filter is used for the adaptive algorithm.